1. Field of Invention
The present invention relates generally to methods and apparatus for use in disk drives for computer systems. More particularly, the present invention relates to methods and apparatus for reducing the vibration of high frequency resonance modes associated with seeking acoustics in disk drives for computer systems.
2. Description of the Relevant Art
The reduction of noise, or vibrations, in disk drives is crucial to ensure that performance issues associated with a disk drive may be minimized. When the performance of a disk drive is not at an acceptable level, customer dissatisfaction regarding the disk drive may be significant, and the integrity of data stored on the disk drive may be compromised. By way of example, when a disk drive experiences excessive vibration, a customer may perceive the disk drive to be operating in a faulty manner. Therefore, the magnitude of the vibrations experienced on a disk drive must generally be reduced to acceptable levels.
FIG. 1 is a diagrammatic representation of a disk drive assembly suitable for use in a computer system. A disk drive assembly 102, which may also be known as a head-disk assembly (HDA), includes a platter assembly 104 that is situated on a base plate 103. Platter assembly, as shown, includes a platter 105 and a spindle mechanism 106. Spindle mechanism 106 typically includes a spindle bearing 107 which is coupled to platter 105, or a disk. Data is stored, or otherwise encoded, on platter 104. Platter 104 may contain multiple spokes 108, each of which includes encoded position information. That is, each spoke 108 contains track numbers and patterns to determine fractional positions which relate to the location of a disk drive, or read/write, head 120 with respect to platter 104.
Disk drive assembly 102 also includes an actuator assembly 114. Actuator assembly 114 includes an actuator 118 which supports disk drive head 120. Actuator assembly 114 is arranged to move disk drive head 120 to different positions over platter 105 such that data may be retrieved from or stored to different data-carrying sectors of platter 105. In general, when disk drive head 120 is to be moved, torque is generated to pivot or otherwise move actuator assembly 114 by a motor assembly 122. Motor assembly 122 is generally mechanically coupled to actuator assembly 114 through an actuator bearing 124.
Actuator motor assembly 122 often includes a coil structure and a magnetic field which surrounds the coil structure, as will be appreciated by those skilled in the art. In other words, actuator motor assembly 122 typically includes a voice coil motor (VCM). By passing current through the coil structure in a particular direction and for a specified length of time, actuator assembly 114 may be moved, e.g., pivoted, such that disk drive head 120 is positioned over a specific portion of the platter 105. The pivoting of actuator assembly 114 to position disk drive head 120 in a desired position is generally known as a "seek."
A spindle bearing 107, which is coupled to a spindle motor (not shown), allows platter 105 to spin with respect to base plate 103. Typically, noise is associated with the rotation of platter 105. Specifically, motor noise associated with a spindle motor, i.e., "spindle noise," contributes to idle acoustics, or acoustics which are present while platter 105 is spinning. The amount of idle acoustics increases as the spinning speed of platter 105 increases. Further, if spindle bearing 107 is not perfectly circular, spindle bearing 107 may further contribute to idle acoustics.
Although the level of idle acoustics in a disk drive assembly may vary, e.g., the level of idle acoustics may depend upon the mechanical design of the disk drive assembly, idle acoustics are typically in the range of approximately 35 decibels (dB) to approximately 40 decibels when the platter spinning speed is approximately 4000 revolutions per minute (RPM). By way of example, idle acoustics in the range of approximately 37 dB to approximately 38 dB are typically the market requirement for 5.25 inch disk drives which have a spindle speed of approximately 4000 RPM.
Acoustics associated with seeking processes, referred to herein as "additional seeking acoustics," are additive with respect to idle acoustics. In other words, acoustics are affected by both idle acoustics and additional seeking acoustics. Generally, the "sum" of idle acoustics, which are caused by the spinning of platter 105, and additional seeking acoustics, which result from the performance of a seek, is considered to be the overall seeking acoustics.
Additional seeking acoustics are typically the result of disk drive structural vibration induced by the seek current. Current is sent through the VCM, i.e., the VCM that is a part of motor assembly 122, to create a torque which is applied to actuator assembly 114 in order to move actuator assembly 114. FIG. 2a is a block diagram which illustrates a conventional system used to generate a torque starting with a current command that is provided by a seek program. A current command 204 is sent to a digital-to-analog (D/A) converter 206, or a pulse width modulator (PWM) to produce a control voltage 207 which is proportional to current command 204, numerically. Current command 204 is sent to D/A converter 206 which, in turn, feeds control voltage 207 into a power amplifier 208, in response to a seek command, or a request to move an actuator assembly in order to position a disk drive head. Power amplifier 208 processes, e.g., amplifies, current command 204 to generate a motor input current 212, or a seek current.
Motor input current 212 is effectively arranged to produce a torque 220 that causes an actuator to move. Specifically, motor input current 212 causes an actuator motor 216 to create torque 220 that moves an actuator to a desired location. Torque 220 may cause the actuator to accelerate, decelerate, or move at a constant velocity. In other words, motor input current 212 is arranged to generate torque 220 that causes an actuator to rotate to a desired position for the actuator as required by a seek command. The profile of motor input current 212 is dependent upon the profile of current command 204.
In order to move an actuator, e.g., actuator assembly 118 of FIG. 1, efficiently to a desired position, the amount of current sent to the VCM is often adjusted substantially instantaneously, as will be described in more detail below with respect to FIGS. 2b and 2c. While the level of noise associated with the VCM during a seek may be widely varied, the level of noise, i.e., the additional seeking acoustic, is typically in the range of approximately 5 dB to approximately 10 dB in sound power, as for example approximately 7 dB.
In general, acceptable levels of overall seeking acoustics in disk drive assemblies are determined based upon what is considered to be tolerable by customers who use the disk drive assemblies. For 5.25 inch disk drives, an overall seeking acoustic which is no more than approximately 45 dB is generally considered to be acceptable, although an overall seeking acoustic which is less than approximately 40 dB is preferred. However, as overall seeking acoustics are often in the range of approximately 45 dB to approximately 50 dB, many disk drives fail to meet acceptable levels of noise during seek operations. Failure to meet acceptable levels for overall seeking acoustics may lead to disqualification of disk drives by customers, as well as performance issues related to the disk drives.
Additional seeking acoustics result from large changes of amplitude in a motor input current which is used to create a torque on an actuator motor. As will be appreciated by those skilled in the art, changes in seek current are correlated to the amount of noise associated with a seek command. Specifically, a higher level of noise is generally attributed to a more rapid change in a current level.
FIG. 2b is a graphical representation of a relationship between a current command and time. A current command or a series of current commands, as for example current command 204 of FIG. 2a, is provided as an input to D/A converter or a PWM which is in communication with a power amplifier. Current command has a profile 240 which effectively has an infinite "beginning" slope 244, or an acceleration phase. Beginning slope 244 is approximately infinite due to the fact that a rapid increase from zero current to a current level which saturates a power amplifier allows the seek time, i.e., the time associated with a seek operation, to be minimized. The power amplifier is operated at saturation, or in saturation mode, to minimize the seek time associated with performing a seek. In addition to beginning slope 244, current command profile 240 also has a "change in polarity" section 248, followed by an ending deceleration section 252, both of which are associated with gradually returning profile 240 to a level of zero current, thereby ending the seek.
With reference to FIG. 2c, the seek current that is sent to an actuator motor in response to the current command of FIG. 2b, will be described. An input current profile 260 is effectively a seek current, or a forcing function which causes an actuator to move. A seek current is arranged to create a bang-bang seek, or a near bang-bang seek. Such a seek is a seek in which maximum current levels are used to create maximum torque levels to efficiently move an actuator. Input current profile 260 is generated when current command profile 240 is passed through a D/A converter, or a PWM, as well as a power amplifier, and is used to create a torque in an actuator motor. A beginning section 264 of input current profile 260 is "rugged," i.e., has a relatively large slope. As shown, beginning section 264, which is typically the one-third stroke seek, has a substantially exponential shape, after an initial sudden change of slope 265. The ruggedness of beginning section 264 signifies an abrupt change in the motor input current which initiates vibration of the disk drive in addition to the desired actuator movement. Input current profile 260 reaches a maximum value 266 which corresponds approximately to the saturation level for the power amplifier which produces input current profile 260, given current command profile 240 of FIG. 2b. A "polarity reversal" section 268 of input current profile 260 signifies a transition from acceleration to deceleration. An ending section 272 signifies a final deceleration which causes the actuator to decelerate and eventually return to rest.
Mechanical solutions are often used in order to reduce overall seeking acoustics. Most mechanical solutions use dampers to damp out vibrations. One common mechanical solution that is used to reduce the overall seeking acoustics involves the use of a foam damping layer. A foam damping layer is mounted on surfaces that emit sound waves, as for example on the top cover of a disk drive. Such a foam damping layer is arranged to absorb energy and, therefore, reduce vibrations on the overall disk drive. However, although the use of a foam damping layer is effective to reduce overall seeking acoustics, the use of a foam damping layer reduces overall seeking acoustics by no more than approximately 2 dB. Further, the use of a foam damping layer is expensive, as it is an additional part that is added to a disk drive, and also requires additional manufacturing time to mount the foam damping layer on the disk drive. As such, the use of a foam damping layer is often not desirable, since the costs associated with the foam damping layer are relatively high given the reduction of overall seeking acoustics that is possible using the foam damping layer. However, foam damping layers are often still used to at least slightly reduce overall seeking acoustics.
Another common mechanical solution which is intended to reduce overall seeking acoustics involves the use of rubber pieces within a disk drive. Specifically, rubber pieces are placed within the disk drive between the actuator motor, or the VCM, and the top cover of the disk drive to reduce the relative motion between the VCM and the top cover. Hence, the rubber pieces are intended to reduce overall seeking acoustics by absorbing energy. As was the case with using a foam damping layer, the rubber pieces also do not significantly reduce overall seeking acoustics. For example, the use of rubber pieces generally does not reduce overall seeking acoustics by more than approximately 1 dB. In addition, the use of rubber pieces is expensive due to the fact that the cost associated with creating the rubber pieces, as well as the time associated with accurately positioning the rubber pieces between the VCM and the top cover, is significant with respect to the reduction of overall acoustics attributed to the use of the rubber pieces. Therefore, the use of rubber pieces to absorb energy may be considered to be a less than desirable solution to the problem of reducing the overall seeking acoustics in a disk drive.
Notch filters are also conventionally used to reduce overall seek-induced vibrations in a disk drive. Although notch filters are mostly used to remove actuator resonances for read/write purposes, notch filters reduce seeking acoustics as well. In particular, notch filters are used to shape the current which used by a VCM to generate a torque, i.e., the seek current. As will be appreciated by those skilled in the art, within the audible frequency range, which ranges from approximately 50 Hertz (Hz) to approximately 10 kiloHertz (kH), there are approximately ten resonant modes. Each notch filter is arranged to notch out, or remove, a particular resonant mode. Since each notch filter used increases the level of instability in the overall disk drive, or, more particularly, the servo arrangement associated with the disk drive, only a few resonant modes may be removed from the overall seeking acoustics. It has been observed that no more than three or four notch filters which are arranged to remove three or three resonant modes may be used without significantly affecting the stability of the overall disk drive. As such, only three or four resonant modes may be selected as being resonant modes which are to be notched out. Therefore, since most resonant modes may not be notched out, overall seeking acoustics are not substantially reduced using notch filters. Further, the use of notch filters often increases the seek time associated with performing a seek.
Many techniques which are used to reduce vibrations in general physical systems may also be applied to disk drive systems in order to reduce overall seeking acoustics in the disk drive systems. One technique which is used is known as current shaping. Current shaping techniques process a current command in order to "round out" the resultant real current. That is, current shaping techniques attempt to smooth out abrupt changes in slope. Such current-shaping techniques are often used to reduce the residual vibrations in a given system. One current-shaping techniques is an "input shaper," which is described in U.S. Pat. No. 5,638,267, issued Jun. 10, 1997, which is incorporated herein by reference in its entirety.
Input shaper techniques generally identify some resonant modes, or frequencies, and essentially remove the frequency components from the current commands. such that a real current is characterized by a relatively smooth curve. In other words, input shaper techniques are arranged to process current commands, using what is commonly known as an "input shaper filter," such that certain resonant frequencies are removed from the input current commands. In some cases, there are only a few resonant frequencies which are not excited by the resultant real current, i.e., some resonant frequencies remain excitable. To effectively cancel out selected resonant frequencies using an input shaper, a knowledge of the resonance modes of the system being controlled is required.
Input shaper techniques typically do not cause system instability. Hence, input shaper techniques may be used to remove a relatively high number of resonant frequencies. However, input shaper techniques are not readily adaptable for use in disk drive technology. That is, modifying input shaper techniques for use in disk drive systems is often complicated, since servo code associated with disk drive systems is arranged to perform a phase-plane seek, as will be appreciated by those skilled in the art, while input shaper techniques are time-based. As reconfiguring input shaper techniques to perform a phase-plane seek may be complicated and time-consuming, and only selected resonant modes are rendered unexcitable, i.e., the overall seeking acoustics may not necessarily be reduced to an acceptable level, the use of such current shaping techniques in disk drive systems may not be desirable. Additionally, conventional current shaping, and input shaping, techniques also require knowledge of the resonant modes in a particular disk drive system.
Other current shaping techniques involve the calculation of velocity errors, i.e., differences between a desired trajectory for an actuator and an actual trajectory in distance and velocity space. FIG. 3 is a graphical representation of a desired seek trajectory and an actual seek trajectory. As shown, a desired seek trajectory 304 and an actual seek trajectory 308 for a disk drive head coupled to an actuator are graphically represented in a distance-velocity domain. That is, desired seek trajectory 304 and actual seek trajectory 308 represent velocities plotted versus distance, where the distance is the distance of the disk drive head from a desired location, e.g., the target of a seek operation.
Velocity errors 312 vary as a disk drive head moves with respect to a desired location. By way of example, velocity error 312, which is measured near the beginning of a seek, is substantially greater in magnitude than velocity error 312b, which is measured near the middle of a seek. Near end 316 of a seek, there is substantially no velocity error, as shown. Hence, near end 316 of the seek, the disk drive head is effectively on track, or at its desired location.
Velocity errors 312, as mentioned above, are used in some current shaping techniques. In particular, velocity errors 312 are used in current shaping techniques which are arranged to reduce the overall seeking acoustics associated with the deceleration portion of a seek. A current command used for such a current shaping technique is often expressed as follows: EQU CurrentCommand=Feedforward+K1*VelocityError+Forces
where the "feedforward" term includes data generated during the acceleration portion of a seek, and the "forces" term includes such forces as a calibrated bias force and the bias force associated with an estimator. The "velocity error" term reflects the difference between desired seek trajectory 304 and actual seek trajectory 308, and is generally measured in terms of tracks per control interval. K1 is a constant gain which is arranged to derate a velocity gain, and is typically chosen to minimize the velocity error and, hence, the amount of noise, associated with end 316 of the seek. K1 is constant in that once a value for K1 is set, that value of K1 is not altered during the creation of a seek current which is generated using the current command expression above.
In general, if K1 is chosen to minimize the velocity error at end 316 of seek, then, as shown, a beginning deceleration portion 320, which corresponds to change of polarity section 268 of FIG. 2c, is characterized by substantially sinusoidal-type oscillations. In other words, when K1 in the current command expression is chosen to minimize velocity error at end 316 of seek, deceleration portion 320 is characterized by multiple, relatively abrupt, changes in slope that result from a bandwidth that is too high. The abrupt changes in slope result from the variation in velocity error increases when the absolute value of the velocity increases, as is typically the case in the middle of a seek. A larger variation in velocity error results in a larger variation in command current, and is a source of acoustic noise. As described above, abrupt changes in slope in seek current and, hence, an actual seek trajectory, cause an increase in additional seeking acoustics and, as a result, the overall seeking acoustics.
Alternatively, if K1 is chosen to reduce the sinusoidal-type oscillations in the actual seek trajectory, then the velocity error at end 316 of the seek may be relatively high. For example, the disk drive head may never accurately reach its desired location, e.g., the on-track performance of the overall disk drive may be unacceptable. In some cases, the seek time required for the disk drive head to reach its desired location may be high, even when on-track performance is considered to be acceptable. As accuracy is crucial and shorter seek times are more efficient than longer seek times, a relatively long seek time is often considered to be undesirable
In view of the foregoing, what is desired is a method and an apparatus for efficiently and effectively reducing the overall seeking acoustics in a disk drive system. More particularly, what is desired is a method and an apparatus for efficiently and effectively reducing the additional seeking acoustics associated with the deceleration portion of a seek operation.